The invention is concerned with a method for improving the `compatibility` of the bandwidth-reduced signal in systems using a coding method that performs temporal subsampling of the signal. Examples of bandwidth reduction systems to which this could be applied are given in our UK patent applications GB-A-2221367 and GB-A-2188509 and in IEE conference publication no: 293. IBC, 1988, September 1988 which contains various papers on High Definition MAC (HD-MAC). Such systems generally use the principle of digitally assisted television (DATV), in which the picture information is sent as an analogue signal but a digital signal is also transmitted whose task is to control the high definition decoder.
The invention will be described with reference to the so-called `40 mS branch` coding algorithm which is described in the aforementioned conference publications, for example in the paper therein entitled `motion compensated interpolation techniques for HD-MAC` by J. P. Arragon, F.
Fonsalas, M. Haghiri, although it can be applied to any branch which works by temporally subsampling the video signal (such as the 80 ms branch described in GB-A-2221367 and the Arragon et al. paper). It may be applied to a bandwidth reduction system having only one coding method or `branch`, or one in which several branches are available and are switched on a block-by-block basis such as is disclosed, for example in WO88/00783.
Before describing the invention in detail, the operation of the 40 mS coding branch will be reviewed. The 40 mS branch encoder spatially pre-filters the incoming video signal and subsamples alternate fields; in a 50 Hz television system the sub-sampled images are separated in time by 40 mS, hence the name given to this branch. Motion vectors are derived at the encoder to indicate how the images that were not sampled may be derived from the adjacent sampled images. The subsampled video signal is then transmitted to the decoder, where the alternate subsampled images are reconstructed using a spatial interpolator. The non-transmitted images are then derived by motion-compensated temporal interpolation.
In a simple embodiment of a 40 ms branch, the subsampled pixels in each alternate image are transmitted over two field periods; half the samples being transmitted in each field. FIG. 1 shows the pixels sampled by the branch and the positions at which they are transmitted. The samples on odd lines of the original field 1 are shown being transmitted in the first field and those from even lines in the second field. This produces a compatible signal that appears to `judder` at 25 Hz, since alternate fields of samples are transmitted one field period later than they should be.